Background / Rationale: Venous thrombus embolism (VTE), which includes deep vein thrombosis (DVT), and its complications are a significant source of morbidity and mortality among Americans; and have increased in the Veteran population over the last decade. A recent study of almost 500,000 surgeries performed at the Department of Veterans Affair Medical Centers found that over half of the patients developed VTE within 90 days after surgery. Along with the potentially fatal complication of pulmonary embolism, DVT frequently leads to a significant long-term complication for which we have no specific therapy, post-thrombotic syndrome; which causes debilitating swelling, pain, and leg ulceration in 25-60% of DVT patients. Common risk factors for DVT include: cancer, major trauma, surgery, paralysis, prolonged periods of immobility, and older age. Deployed military personnel are at an increased risk due to prolonged air and ground transport, dehydration, tobacco use, and extended immobility during hospitalizations for severe injuries. Current therapies rely on anticoagulants to treat DVT, which do not resolve existing blood clots; but only prevent further clot development. Thrombus resolution is a critical factor in the pathogenesis of post-thrombotic syndrome since incomplete thrombus resolution can result in obstruction of flow and loss of venous valve function. Clinical studies show that patients with more rapid thrombus resolution have a better prognosis than those patients whose thrombus resolves much slower. At present, the cellular and molecular mechanisms involved in DVT are poorly understood, and there currently is no therapy to accelerate this process. Objectives: Using clinically relevant experimental models of DVT, we have uncovered a critical molecular pathway that functions as a key modulator of inflammation during venous thrombus resolution. A comprehensive picture of interconnected cell-mediated molecular processes that orchestrate a precise inflammatory program is starting to emerge. Our research plan proposes to define this pathway by (1) determining mechanisms by which plasminogen activator inhibitor type 2 (PAI-2) deficiency modulates early acute DVT to accelerate venous thrombus resolution, and (2) determining the role of plasmin-generated fibrin degradation products (FDPs) in regulating inflammatory macrophages during venous thrombus resolution. Methods: Studies will utilize genetically deficient mice in experimental models of DVT that accurately mimic many of the clinical and pathophysiological features observed in human DVT. Venous thrombi will be analyzed by immunohistochemistry, flow cytometry, mRNA and protein analyses for molecular indicators of inflammation and thrombus resolution. Proteolytic pathways will be investigated using ex vivo thrombolysis assays, cellular clot lysis assays, and ex vivo cell culture. Morphometric analyses and biomechanical assays will assess vein wall injury. The translational potential of these findings will be tested in human cells and in preclinical models. Findings/Results: Novel molecular mechanisms that modulate inflammation during venous thrombus resolution will be identified and the potential for therapies based on these mechanisms tested in preclinical models. Status: This is a new project arising from substantial supportive preliminary data from a previous VA Merit Award. Impact: The knowledge gained from these studies has strong potential for clinical translation since specific antagonism of inhibitors of venous thrombus resolution or resolution-promoting agonists could present safe and effective therapies for accelerating this process, in combination with anticoagulants, provide an improved treatment for reducing venous pulmonary embolism and post-thrombotic complications from DVT in our Veteran population.